Dock levelers are commonly used at loading docks to bridge the gap between a parked vehicle and the loading dock. An approaching vehicle comes to rest near the loading dock, but with some open space between the vehicle's cargo bed and the loading dock's edge. The dock leveler is able to engage the vehicle's bed to provide a continuous driving surface to the dock and thereby facilitate loading and unloading. Given the size and weight of fork-lifts, this driving surface should be smooth and able to support substantial weight.
Dock levelers typically include a frame formed within the pit of a loading dock and a deck pivotally attached to an end of that frame. The deck is movable from a “stored” position, also known as a dock level position, wherein the deck is even with the warehouse or building floor, and a range of operating positions both above and below this position. The range of operating positions is useful to engage vehicles of different bed heights. The range is also useful to maintain engagement with the bed as its height fluctuates with the reaction of the vehicle's suspension during loading and unloading.
At the front end of the deck is a lip pivotally connected to the front end of the deck for movement between a pendant, or stored, position and an extended position. In the extended position, the lip may bridge the gap between the deck and the bed of the parked vehicle to form a continuous driving surface. Typically, in this configuration, the engagement between the vehicle bed and the lip is the only thing supporting the deck against falling.
During engagement, the lip and deck are able to ride with the vehicle bed as that bed floats, for example, as different downward forces are exerted on the vehicle's bed and the vehicle's suspension system. Although the freedom of movement is desirable, it presents some serious problems.
With certain levelers, if the vehicle prematurely pulls away, and in particular with a load on it, the support for the dock leveler is removed, and a free fall condition may be created. The entire deck may fall hitting the pit or the dock leveler frame, potentially causing injury to any fork truck operator on the deck, and secondarily damaging equipment and cargo. In addition to the damage from the impact, the fallen deck plate is left at a steep sloping angle creating a further potential hazard at the dock's edge.
Attendant to preventing free fall, manufacturers have devised numerous techniques for preventing this uncontrolled fall of a deck from premature departure. Different types of dock levelers offer different types of free fall protection.
Mechanical levelers typically use springs to raise the deck from the stored position to a preparatory position. A “hold-down” device normally holds the leveler down against the upward bias of these springs, but may be released to raise the deck. Once the deck reaches the preparatory position, the lip is moved to its extended position, and a subsequent downward rotation of the deck will place the lip on the bed of the vehicle. With mechanical levelers, personnel must “walk down” the deck of the dock leveler to a position wherein the lip rests on the bed of the vehicle.
To limit free fall, mechanical levelers (as well as other levelers such as air operated levelers) use so-called ‘safety legs,’ i.e., legs that serve to stop the leveler from free falling beyond a certain position. Typically, the safety legs extend from the bottom of the deck and engage a fixed-height pedestal disposed in the pit or on the frame. Contact between the safety leg and the pedestal arrests any further downward movement of the deck. Thus, if a vehicle prematurely departs with a load on the deck, the deck will only “free fall” until the safety legs engage their pedestals. In a mechanical leveler application, if there is no load on the deck, the upward bias of the springs prevents downward movement of the deck. But if there is a load on the deck during premature departure, the weight of that load may cause the free fall of the deck.
There are various safety leg designs. And while these various designs offer different approaches and different advantages over one another, they are all incomplete. One limitation is that safety legs block the deck of a dock leveler from freely floating to below dock positions, with changes in vehicle bed height. If, for example, the weight of a fork truck forces the vehicle bed and deck downward, as occurs increasingly more frequently during loading and unloading of air ride suspension vehicles, the safety legs will engage the stopping pedestal and block the deck from further downward movement. If the vehicle bed moves further, e.g., as a result of the fork truck's weight entering the vehicle, then the lip engaging that bed will form a very steep angle from the bed to the deck. In essence, the deck will be suspended above the vehicle bed, by the length of the safety legs and pedestals, as the lip is acutely angled.
In this position, a fork truck may be forced to negotiate this steep slope during loading and unloading. As a result, the steep slope on the lip acts as a backstop or impediment to fork truck movement and, indeed, may prevent the fork truck from driving back onto the deck entirely. This condition, typically referred to in the industry as “stump out” is an inconvenience, and represents a potential safety hazard to the fork truck operator who does not notice the significant slope of the lip. Stump out can endanger dock personnel and damage the lip and lip hinge.
A number of different safety leg designs have been implemented to try and address stump out. Ultimately none have been successful. To allow the deck of the dock leveler to freely ride with the vehicle bed without being limited in downward movement, retractable safety legs have been designed. The legs are retracted rearwardly during normal dock leveler operation, including below-dock float of the deck. But once a free fall condition on the deck is detected, the legs are to be released from the retracted position and moved into place for engaging the pedestal. While such designs may function in theory, they fall short of performing that task completely and reliably.
Retractable safety leg attempt to balance two competing goals. They retract to allow free movement of the deck during normal operation; yet, they should also deploy fast enough to prevent complete free fall of the deck during premature departure of a vehicle. In theory, these safety leg designs should require a deployment mechanism that is faster than the deck's free fall. In practice, they do not. The various mechanical and other safety leg release mechanisms fail to prevent free fall in many situations, because either the mechanisms take too long to detect free fall or the safety legs take too long to release to prevent free fall.
In an attempt to skirt around this race to the bottom, i.e., where a safety leg must deploy in time to prevent complete free fall, some safety leg designs will allow a partially retracted leg to still engage a pedestal, for example, by placing multiple engaging stops (or stairs) on the pedestal. The safety leg can engage the stop nearest to it, even when in the retracted position. These and other similar designs attempt to lessen the deployment distance of the safety leg.
Still other designs attempt to trigger deployment before the vehicle has actually pulled away from the lip of the dock leveler. In U.S. Pat. No. 6,276,016 (“Safety Leg System For a Dock Leveler”), Springer described a system having a leg control member that will sense a vehicle prematurely pulling away from a dock and release a safety leg biased toward the deployed position, while a portion of the lip is still in contact with a vehicle bed. Thus, in theory, the release of the safety leg may happen before the vehicle has even moved completely away from the lip's edge. This design and other designs attempt to reduce the amount of time required between complete free fall and full deployment. Other designs sense lip fall, for example, by relying upon lip movement to trigger safety leg deployment.
Still other designs rely upon the speed of a deck's fall. One such design includes a safety leg attached to a roller, where under normal operating conditions, the roller rides along an engagement surface. When enough downward force is applied to the deck, for example, during free fall, the roller is retracted and the safety leg is exposed for engagement in a multiple-locking-position pedestal.
None of the these designs are satisfactory, because inevitably they all limit the extent to which a deck can travel below the dock position and still have protection against free fall. It would be advantageous to be able to provide a safety leg, or similar, system that does not suffer from the disadvantages of stump out and still allows for full range free float on the deck during normal operation.